If an electric potential is established between base and cover layers, then a cathode current is discharged through the cover layer provided the layers' thicknesses are correctly dimensioned. Although this type of cathode is, in theory, of relatively simple construction and capable of providing remarkable advantages compared with a barium cathode, it has encountered considerable difficulties in practical application. To obtain a useful flow of electrons through the insulation and cover layers, these layers must be extremely thin, and, at the same time, the insulation layer must be able to withstand a high electric field intensity without breakdown, flashover or sparking.
In an earlier publication, K. M. Tischer determined that a field intensity level of approximately 5.times.10.sup.6 V/cm is required. On the other hand, in the "MEAD" research, an emission of approximately 10 lambda/cm.sup.2 is disclosed as attainable.
An aluminum oxide layer (Al.sub.2 O.sub.3) is generally employed as an insulation layer having sufficient resistance and strength characteristics. Unfortunately, aluminum oxide, being a good adsorbent, is quite hygroscopic and because of this, is virtually useless as a practical matter if it operates in a region wherein water vapor may be present.
Thus other materials have been generally preferred. For example, European Patent 262 676 A2 discloses an insulation layer that is composed of special organic substances or a combination of an organic and an inorganic layer. Nevertheless, a need exists for a tunneling cathode wherein a sufficiently thin insulating layer coexists with the high electric field intensity requisite to produce a tunneling effect which more nearly approaches that theoretically possible.
The following is a list of publications which are relevant to the state of the art and the background of the instant invention, such publications being incorporated herein by reference:
C. A. Mead, J. Appl. Phys. 32, 646 (1961)
J. P. Spratt, R. P. Schwarz, and W. M. Kane, Phys. Rev. Letter 6, 341 (1961)
H. Thomas, Z. Physik 147, 395 (1957)
W. G. Spitzer, C. R. Crowell, and M. M. Atalla, Phys. Rev. Letters 8, 57 (1962)
C. A. Mead, Phys. Rev. Letters 8, 56 (1962)
J. C. Fisher and I. Giaver, Appl. Phys. 32, 172 (1961)
J. T. Advani, M.S. Thesis, MIT (May 1961)
R. M. Handy, Phys. Rev. 126, 1968 (1962)
K. M. Tischer, Telefunken AG, Rohrenwerk Ulm/Donau
C. E. Horton and J.W. Hall, GE-Technical Inf. Series
C. Mead, Phys. Rev. 128, 2088 (1962)
W. Haas and R. Johannes, Brit. J.appl. Phys. 14, 286 (1963)
R. Fowler and L. Nordheim, Proc. Soc. A 119, 173 (1928)
J. Bardeen, Phys. Rev. 71, 717 (1947)
H. Kanter and W. A. Feibelman, J.appl. Phys. 33, 3580 (1962)